In engines that perform lean combustion, such as diesel engines, an operation range in which an air-fuel mixture having a high air-fuel ratio (lean atmosphere) is caused to combust accounts for most of the entire operation range, thus causing concern regarding the emission of a relatively large amount of nitrogen oxides (hereinafter referred to as NOx). As a countermeasure, it is known to provide an EGR (Exhaust Gas Recirculation) apparatus that recirculates part of the exhaust gas into the intake path (e.g., see PTL 1 and PTL 2). Specifically, the in-cylinder oxygen concentration and oxygen density are lowered by recirculating exhaust gas into the cylinders. This lowers the combustion temperature (flame temperature) during combustion in the combustion chamber, thereby suppressing the production of NOx and improving exhaust emissions.
On the other hand, if the air-fuel mixture undergoes incomplete combustion during combustion in the combustion chamber, smoke is produced in the exhaust gas, thus leading to worsening of exhaust emissions. As a countermeasure for reducing the amount of smoke that is produced, it has been proposed in PTL 3 described below, for example, to execute auxiliary combustion in the compression stroke of the engine, and thereafter execute main injection so as to achieve a diffusion combustion state. In other words, causing the combustion in the auxiliary injection to be premixed combustion overcomes oxygen deficiency in the combustion field, thus suppressing the production of smoke.